Patent GB-1187377-A (GB)
Title: Improvements in or relating to Proximity Detectors
IPC: G01V3/10
Filed: 1967-04-14
Abstract: 1,187,377. Transistor oscillator circuits. COMPAGNIE DES COMPTEURS. 14 April, 1967 [15 April, 1966], No. 17340/67. Heading H3T. [Also in Division G1] In a transistor oscillator the base of the transistor T is connected to a tuned circuit comprising a proximity detector winding L and a capacitor C 1  in parallel therewith and also to a rectifier D1, and a negative feedback resistance R 1  is connected to the emitter of the transistor T, the arrangement being such that proximity of a conductive mass to the winding L causes a reduction in the amplitude of oscillations at the collector of transistor T. The circuit shown in Fig. 2 also includes a coupling capacitor C 2 , a bias resistor R 2  and a smoothing capacitor C 3  and is energized by a D.C. power source V via a load circuit RC. Capacitor C 1  may be connected to a tapping on winding L different from that to which capacitor C 2  is connected, Fig. 4 (not shown), and the negative feedback resistance may be connected to a point in the capacitive arm of the tuned circuit, Fig. 4a (not shown), especially if the winding L is remote from the rest of the circuit. Reducing switching hysteresis.-Point M may be connected to a tapping on winding L between the lower end of the winding and point A<SP>1</SP>, or the arrangement shown in Fig. 6 may be used in which the end 6 of resistor R 4  may be connected to either point M or A<SP>1</SP>. If 6 is connected to M, diode D1 may be omitted. Increasing switching hysteresis.-The negative feedback resistance R 1  may comprise two series resistors, one of which is shunted by a diode, or the transistor collector may be connected to the upper end of winding L via a current limiting network of resistors and diodes, Fig. 8 (not shown), the A.C. impedance of which increases with the voltage across winding L. Compensating for temperature variations.- The bias resistor R 2  may be connected to a point in a resistance voltage-divider chain connected between points S and M and including an N.T.C. thermistor (R 9 , Fig. 9, not shown). The circuit shown in Fig. 6 may be provided with a temperature-compensating diode (D6, Fig. 10, not shown) connected in series with resistor R 4 . Fig. 11.-The circuit shown makes it possible to reduce the voltage across winding L for a given output power so as to reduce the repulsion exerted on the conductive mass. Circuits giving a proportional output.-An amplitude of oscillation proportional to the Q factor of the detector winding L is achieved by connecting a voltage-dependent impedance in parallel with the tuned circuit. The impedance may comprise a capacitor, resistor and Zener diode in series or may be a network of resistors and diodes, Figs. 14 to 17 (not shown), having a point (D) connected to a fixed positive D.C. potential or to the transistor collector. Detection by variation of mutual inductance.- The proximity of a conductive mass may vary the coupling between winding L and a second winding (L<SP>11</SP>) connected in series with either the feedback resistor R 1  or the transistor collector, Figs. 18 and 19 (not shown).